# FLT3 Receptor: Internal Tandem Duplications (ITD), TKD Mutations, and Targeted AML Therapeutics


## Key Takeaways

- *FLT3* mutations, specifically internal tandem duplications (ITDs) in the juxtamembrane domain and tyrosine kinase domain (TKD) point mutations (e.g., D835Y), are prevalent in approximately 30% of adult acute myeloid leukemia (AML) cases, conferring constitutive kinase activation and driving leukemogenesis.
- FLT3-ITD mutations are strongly associated with a poor prognosis, increased relapse risk, and higher blast counts, with the mutant-to-wild-type allelic ratio serving as a critical prognostic factor.
- Targeted therapies, including first-generation (e.g., midostaurin) and highly selective second-generation FLT3 inhibitors (e.g., gilteritinib, quizartinib), are approved for FLT3-mutated AML, demonstrating improved outcomes in both newly diagnosed and relapsed/refractory settings.
- Resistance to FLT3 inhibitors can emerge through on-target mutations (e.g., secondary TKD mutations) or off-target pathway activation, necessitating ongoing research into combination strategies and novel therapeutic agents.
- Molecular diagnostic methods such as PCR-based fragment analysis for ITDs and Sanger sequencing or next-generation sequencing (NGS) for TKD mutations are crucial for identifying these alterations and guiding treatment decisions.

---

## Executive Summary & Key Metadata

The *FLT3* gene (Fms-related receptor tyrosine kinase 3; also known as CD135, FLK2, or STK1) encodes a class III receptor tyrosine kinase (RTK) that is a central regulator of early hematopoiesis, dendritic cell development, and B-lymphoid progenitor commitment [1, 2, 3, 4]. Somatic mutations in *FLT3*—most notably internal tandem duplications (ITDs) in the juxtamembrane (JM) domain and point mutations in the tyrosine kinase domain (TKD), such as D835Y—are among the most frequent molecular aberrations in acute myeloid leukemia (AML), occurring in approximately 30% of adult cases [1, 5, 6, 7, 8, 9]. These mutations confer constitutive, ligand-independent kinase activation, driving aberrant proliferation and survival of leukemic blasts, and are associated with poor prognosis and increased relapse risk [1, 2, 10]. The clinical significance of *FLT3* has driven the development of a robust pharmacopeia of small-molecule inhibitors, including first-generation agents (e.g., sorafenib, midostaurin) and highly selective second-generation inhibitors (e.g., gilteritinib, quizartinib) [3, 4, 5, 8]. This reference manual provides an exhaustive, publication-grade overview of the genomic architecture, structural biology, signaling networks, pathogenic mutations, [pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), and bioinformatic resources pertaining to *FLT3*.

| **Attribute** | **Value** |
|---|---|
| HGNC Symbol | FLT3 |
| UniProt Accession | P36888 |
| Representative PDB ID | 4RT7 |
| Chromosomal Locus | 13q12.2 [6, 7] |
| Primary Molecular Function | Class III receptor tyrosine kinase; cytokine receptor for FLT3 ligand (FLT3LG); regulates hematopoiesis, cell survival, proliferation, and differentiation [2, 3, 4] |
| Disease & Pathology Associations | Acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), myelodysplastic syndromes (MDS), mast cell tumors (canine), neuropathic pain [1, 2, 5, 8, 9, 10] |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The human *FLT3* gene is located on the long arm of chromosome 13 at band q12.2 (13q12.2), a region that also harbors the related *FLT1* (VEGFR1) gene, suggesting an ancestral duplication event within the RTK gene family [6, 7]. The gene spans approximately 100 kilobases (kb) of genomic DNA and is oriented on the plus strand. Early mapping studies using fluorescence *in situ* hybridization (FISH) and yeast artificial chromosome (YAC) contigs localized *FLT3* to 13q12→q13, with subsequent refinement to 13q12.2 [6, 7].

The coding sequence of *FLT3* is organized into 24 exons, with the exon/intron boundaries highly conserved among class III RTKs, including *KIT*, *PDGFRA*, and *PDGFRB* [3]. The genomic structure of the downstream portion of the gene (exons 15–24) was characterized by Agnès et al. (1994), who demonstrated that the intron phases and sizes are remarkably conserved across subclass III RTKs, underscoring a common evolutionary origin [3]. The translation initiation codon (ATG) is located in exon 1, and the stop codon is in exon 24. The 3' untranslated region (UTR) contains multiple AU-rich elements (AREs) that mediate mRNA instability, a feature common to cytokine and growth factor transcripts.

### 1.2 Promoter Architecture and Transcriptional Regulation

The *FLT3* promoter lacks a canonical TATA box but contains a high GC content and multiple Sp1 binding sites, features typical of housekeeping and growth factor receptor genes. Transcriptional regulation of *FLT3* is complex and cell-type specific, with distinct cis-regulatory elements controlling expression in hematopoietic stem cells (HSCs), common lymphoid progenitors (CLPs), and dendritic cell (DC) precursors [4, 5]. Volpe et al. (2009) identified a critical enhancer region upstream of the *FLT3* promoter that is bound by the transcription factors SCL/TAL1, RUNX1, and GATA2 in normal hematopoietic progenitors [5]. In leukemic stem cells (LSCs), this enhancer undergoes epigenetic remodeling, leading to aberrant *FLT3* overexpression or silencing depending on the mutational context [4, 5].

The promoter also contains binding sites for the ETS family transcription factors (e.g., PU.1, FLI1), which are essential for myeloid and B-lymphoid development. PU.1 has been shown to directly transactivate the *FLT3* promoter in myeloid progenitors, while PAX5, a B-cell commitment factor, represses *FLT3* expression in pro-B cells [4]. This antagonistic regulation ensures that *FLT3* is expressed at high levels in early multipotent progenitors but is downregulated upon lineage commitment to the myeloid or B-cell fate.

### 1.3 Alternative Splicing and Isoforms

Alternative splicing of *FLT3* generates multiple mRNA isoforms, although the functional significance of most remains incompletely characterized. The predominant transcript encodes the full-length, membrane-spanning receptor of 993 amino acids (UniProt P36888). A soluble isoform lacking the transmembrane domain, generated by exon skipping of exon 14, has been detected in human plasma and may act as a decoy receptor for FLT3LG [2]. Additionally, a truncated isoform lacking the kinase insert domain has been reported in leukemic cell lines, though its signaling capacity is diminished [6, 7].

In the context of *FLT3*-ITD, the duplicated region is always in-frame and can range from 3 to over 400 base pairs (bp) in length, typically involving exons 14 and 15 (which encode the juxtamembrane domain) [1, 8]. The ITD can be inserted in tandem at the original locus or, less commonly, at a distant site within the JM domain, leading to a "head-to-tail" duplication. The length and insertion site of the ITD influence the degree of constitutive kinase activation and clinical prognosis, with longer ITDs generally associated with worse outcomes [1, 8].

---

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Structure and Domain Organization

The FLT3 receptor is a single-pass type I transmembrane glycoprotein of 993 amino acids, with a molecular weight of approximately 125 kDa (unglycosylated) and 158–160 kDa (fully glycosylated) [9]. The protein is organized into five distinct structural domains, from N-terminus to C-terminus:

1. **Extracellular ligand-binding domain (residues 1–541)**: Comprises five immunoglobulin (Ig)-like domains (D1–D5). D1–D3 are involved in high-affinity binding to FLT3LG, while D4–D5 are required for receptor dimerization and membrane-proximal flexibility [2, 9].
2. **Transmembrane domain (residues 542–563)**: A hydrophobic α-helix that anchors the receptor in the plasma membrane.
3. **Juxtamembrane (JM) domain (residues 564–603)**: An intracellular, membrane-proximal region that plays an autoinhibitory role in the unliganded state. This is the site of ITD mutations in AML [1].
4. **Tyrosine kinase domain (TKD) (residues 604–956)**: Split into an N-terminal lobe (residues 604–700) and a C-terminal lobe (residues 701–956), separated by a kinase insert domain (residues 762–815). The TKD contains the ATP-binding pocket (P-loop, residues 611–617), the catalytic loop (HRD motif, residues 791–793), and the activation loop (A-loop, residues 829–857) [3, 10].
5. **C-terminal tail (residues 957–993)**: Contains multiple tyrosine residues that serve as docking sites for downstream signaling proteins upon phosphorylation.

### 2.2 Structural Basis of Autoinhibition and Activation

In the unliganded, basal state, FLT3 exists as an autoinhibited monomer. The JM domain adopts a "closed" conformation that inserts into the interface between the N- and C-terminal lobes of the TKD, stabilizing the activation loop in a non-permissive, "DFG-out" conformation [3, 10]. This autoinhibitory interaction is mediated by a conserved tryptophan (W603) and a tyrosine (Y589) in the JM domain, which form hydrogen bonds with residues in the N-lobe.

Upon binding of FLT3LG, the receptor undergoes ligand-induced dimerization, bringing the two TKDs into close proximity. This promotes *trans*-autophosphorylation of critical tyrosine residues in the JM domain (Y589, Y591, Y599) and the activation loop (Y842, Y845). Phosphorylation of the activation loop induces a conformational switch to the "DFG-in" active state, fully opening the ATP-binding cleft and enabling catalysis [1, 3]. The JM domain then dissociates from the TKD, allowing substrate access.

### 2.3 Structural Consequences of FLT3-ITD and TKD Mutations

**FLT3-ITD**: The ITD disrupts the autoinhibitory function of the JM domain. The duplicated sequence (typically 3–100+ amino acids) physically displaces the JM domain from its docking site on the TKD, destabilizing the closed conformation and promoting ligand-independent dimerization and autophosphorylation [1, 10]. Structural modeling studies suggest that ITDs introduce a flexible "hinge" that allows the JM domain to swing away from the kinase domain, even in the absence of ligand [10]. This results in constitutive activation of the kinase and downstream signaling pathways (STAT5, PI3K/AKT, RAS/MAPK) [2, 7].

**TKD point mutations**: The most common TKD mutation is D835Y, located in the activation loop (A-loop). Aspartate 835 is a critical residue that coordinates a magnesium ion in the ATP-binding site and stabilizes the inactive conformation. Substitution with tyrosine (D835Y) or other amino acids (e.g., D835V, D835H, D835N) disrupts the autoinhibitory interactions of the A-loop, favoring the active "DFG-in" state [3, 10]. Other TKD mutations include I836del, N841K, and Y842C, all of which similarly promote constitutive kinase activity [4, 5].

### 2.4 Interactive 3D Visualizer

[Interactive 3D Protein Visualizer: Load FLT3 (PDB: 4RT7)](/tools/protein-structure-viewer?source=direct&pdbId=4RT7)

The representative crystal structure 4RT7 corresponds to the kinase domain of FLT3 in complex with a type II inhibitor, revealing the DFG-out conformation. Users can explore the JM domain, ATP-binding pocket, activation loop, and the structural impact of ITD and D835Y mutations using the embedded molecular viewer.

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## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Canonical FLT3 Signaling in Normal Hematopoiesis

FLT3 is expressed on the surface of hematopoietic stem and progenitor cells (HSPCs), including long-term HSCs (LT-HSCs), short-term HSCs (ST-HSCs), multipotent progenitors (MPPs), common lymphoid progenitors (CLPs), and common myeloid progenitors (CMPs) [4]. Its ligand, FLT3LG, is produced by bone marrow stromal cells and T cells, and exists in both membrane-bound and soluble forms [2, 6]. Signaling through FLT3 is essential for the expansion and differentiation of early B-lymphoid progenitors and dendritic cell (DC) precursors [4, 7, 8].

Upon FLT3LG binding and receptor dimerization, autophosphorylation of tyrosine residues creates docking sites for Src homology 2 (SH2) domain-containing proteins. The major downstream signaling cascades include:

- **PI3K/AKT/mTOR pathway**: Phosphorylated Y589 and Y591 recruit the p85 regulatory subunit of PI3K, leading to activation of AKT and mTOR, promoting cell survival and protein synthesis [2, 7].
- **RAS/MAPK pathway**: The adaptor protein GRB2 binds to phosphorylated Y599 and recruits SOS, activating RAS and the RAF/MEK/ERK cascade, driving proliferation [2, 9].
- **STAT5 pathway**: FLT3 directly phosphorylates STAT5 (via JAK2 or directly), leading to STAT5 dimerization and nuclear translocation, where it transactivates genes involved in cell cycle progression (e.g., *CCND1*, *MYC*) and anti-apoptosis (e.g., *BCL2L1*) [2, 7].
- **PLCγ pathway**: Phospholipase Cγ (PLCγ) binds to phosphorylated Y768 and Y955, generating inositol trisphosphate (IP3) and diacylglycerol (DAG), leading to calcium release and PKC activation [1].

### 3.2 Oncogenic Signaling in FLT3-ITD and TKD Mutants

In FLT3-ITD and TKD-mutant cells, the receptor is constitutively active, leading to ligand-independent, persistent activation of the aforementioned pathways. However, the signaling profile of FLT3-ITD differs qualitatively from that of wild-type (WT) FLT3 or TKD mutants:

- **STAT5 hyperactivation**: FLT3-ITD uniquely and strongly activates STAT5, which is not a major downstream effector of WT FLT3 [2, 7]. Constitutive STAT5 signaling drives expression of *PIM1*, *SOCS2*, and *MYC*, contributing to leukemogenesis.
- **Altered subcellular localization**: FLT3-ITD is retained in the endoplasmic reticulum (ER) and Golgi apparatus, where it can signal in a ligand-independent manner [10]. This aberrant localization also leads to ER stress and activation of the unfolded protein response (UPR), which may contribute to leukemic cell survival [10].
- **Differential phosphorylation kinetics**: Razumovskaya et al. (2008) identified novel phosphorylation sites in FLT3-ITD and D835Y that are not phosphorylated in WT FLT3, including Y572, Y589, and Y591 in the JM domain [1]. These sites create additional docking sites for GRB2 and PI3K, amplifying downstream signaling.

### 3.3 Negative Regulation and Protein-Protein Interactions

FLT3 signaling is tightly regulated by several negative feedback mechanisms:

- **Receptor protein tyrosine phosphatases (RPTPs)**: PTPRJ (DEP-1) dephosphorylates FLT3 at key tyrosine residues, attenuating signaling [1]. Disruption of PTPRJ oligomerization has been shown to counteract FLT3-ITD oncogenic activity [1].
- **Suppressors of cytokine signaling (SOCS)**: SOCS2 and SOCS3 are induced by STAT5 and bind to phosphorylated FLT3, targeting it for proteasomal degradation [7].
- **SHIP1**: The inositol 5-phosphatase SHIP1 is phosphorylated and activated downstream of FLT3, negatively regulating the PI3K/AKT pathway [2]. In FLT3-ITD cells, SHIP1 is degraded via Src kinase-mediated phosphorylation of Y1021, leading to sustained AKT activation [2].
- **CBL E3 ligase**: The adaptor protein CBL binds to phosphorylated FLT3 and ubiquitinates it, promoting receptor internalization and lysosomal degradation [7].

### 3.4 Protein-Protein Interaction Networks

STRING and BioGRID databases list over 50 high-confidence protein-protein interactors for FLT3. Key nodes include:

- **Adaptors**: GRB2, SHC1, GAB2, CBL
- **Kinases**: PI3K (p85α), SRC, JAK2, SYK
- **Phosphatases**: PTPRJ, PTPN6 (SHP1), PTPN11 (SHP2)
- **Transcription factors**: STAT5A/B, RUNX1, PU.1
- **Chaperones**: HSP90, SET (involved in membrane trafficking) [10]

```mermaid
sequenceDiagram
    participant FLT3LG
    participant FLT3 as "FLT3 Receptor"
    participant PI3K as "PI3K/AKT"
    participant RAS as "RAS/MAPK"
    participant STAT5 as "STAT5"
    participant PTPRJ as "PTPRJ (negative regulator)"
    FLT3LG->>FLT3: Ligand binding & dimerization
    FLT3->>FLT3: Autophosphorylation (Y589, Y591, Y599, Y842)
    FLT3->>PI3K: Recruits p85 (via Y589/Y591)
    FLT3->>RAS: Recruits GRB2/SOS (via Y599)
    FLT3->>STAT5: Direct phosphorylation (ITD-specific)
    PI3K->>PI3K: AKT activation → survival
    RAS->>RAS: ERK activation → proliferation
    STAT5->>STAT5: Nuclear translocation → gene transcription
    PTPRJ->>FLT3: Dephosphorylation (negative feedback)
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 FLT3-ITD Mutations

FLT3-ITD is the most common *FLT3* mutation, present in 20–30% of adult AML cases and 10–15% of pediatric AML cases [1, 2, 5]. The ITD occurs in the juxtamembrane domain, encoded by exons 14 and 15, and results in an in-frame duplication of 3 to over 400 base pairs [1, 8]. The duplicated sequence is always inserted in tandem, preserving the reading frame.

**Clinical significance**: FLT3-ITD is associated with leukocytosis, higher blast counts, and a significantly increased risk of relapse and death [1, 3, 10]. The prognostic impact is allele-dose dependent: patients with a high mutant-to-wild-type allelic ratio (≥0.5) have a particularly poor outcome, whereas those with a low ratio (<0.5) have an intermediate prognosis [1, 3]. FLT3-ITD is also associated with an increased risk of extramedullary disease and central nervous system (CNS) involvement [1].

**Disease associations**: FLT3-ITD is found in AML with normal cytogenetics, as well as in AML with t(15;17) (PML-RARA), t(8;21) (RUNX1-RUNX1T1), and inv(16) (CBFB-MYH11) [3, 4]. It is also detected in myelodysplastic syndromes (MDS), where it is associated with leukemic transformation [1, 5]. In canine mast cell tumors, FLT3-ITD has been identified as a recurrent mutation, suggesting cross-species conservation of the oncogenic mechanism [6, 10].

### 4.2 FLT3-TKD Mutations

TKD mutations are less common than ITDs, occurring in 5–10% of AML cases [1, 3, 5]. The most frequent is D835Y, a missense mutation in the activation loop of the TKD. Other TKD mutations include D835V, D835H, D835N, I836del, N841K, and Y842C [3, 4, 5]. These mutations are typically point mutations or small in-frame deletions/insertions within exon 20.

**Clinical significance**: The prognostic impact of TKD mutations is less clear than that of ITDs. Some studies report a neutral or even favorable prognosis, while others associate TKD mutations with an increased risk of relapse [3, 5]. Unlike ITDs, TKD mutations do not typically activate STAT5 signaling to the same extent, which may explain their weaker oncogenic potential [2, 7].

### 4.3 Non-Canonical and Germline FLT3 Variants

Recent studies have identified non-canonical *FLT3* alterations, including:

- **Germline variants**: Gordon et al. (2024) described novel germline *FLT3* variants that lead to somatic "gene rescue" mutations in AML, where the wild-type allele is lost and the germline variant is duplicated [7]. These variants may confer a hereditary predisposition to myeloid malignancies.
- **Gene fusions**: The *Daple-FLT3* gene fusion, involving the coiled-coil domain of Daple (CCDC88C) and the TKD of FLT3, has been identified in leukemia cells [8, 9]. This fusion results in constitutive kinase activation and is responsive to FLT3 inhibitors [9].
- **Single-nucleotide polymorphisms (SNPs)**: The D324N SNP (rs121913489) in the extracellular domain has been associated with a higher risk of myeloid leukemias [10].

### 4.4 ClinVar Classifications and Pathogenicity

ClinVar lists numerous *FLT3* variants with classifications ranging from benign to pathogenic. Pathogenic variants are predominantly:

- **FLT3-ITD**: Not typically cataloged in ClinVar due to their length and insertion site variability, but universally recognized as pathogenic in the context of AML.
- **D835Y/V/H/N**: Classified as pathogenic, with strong evidence of constitutive kinase activation [3, 10].
- **I836del**: Pathogenic, associated with AML and resistance to certain inhibitors [4].
- **N841K**: Pathogenic, located in the activation loop, promotes kinase activity [5].

### 4.5 Clinical Differentials and Diagnostic Considerations

The differential diagnosis of *FLT3*-mutated AML includes other RTK-mutated leukemias (e.g., *KIT*-mutated AML), as well as AML with *NPM1*, *DNMT3A*, or *IDH1/2* mutations, which frequently co-occur with *FLT3*-ITD [1, 3]. Flow cytometric analysis of CD135 (FLT3) expression can aid in diagnosis, with high CD135 expression correlating with *FLT3* mutations [1, 2]. However, CD135 expression is not specific to *FLT3*-mutated AML, as it is also expressed on normal HSPCs and some ALL blasts [1, 3, 8].

Molecular diagnostic methods for *FLT3* mutations include:

- **PCR-based fragment analysis**: For ITD detection, using fluorescently labeled primers flanking exons 14–15, followed by capillary electrophoresis [3, 4].
- **Sanger sequencing**: For TKD mutation detection, targeting exon 20 [3].
- **Next-generation sequencing (NGS)**: Comprehensive panels that detect ITDs, TKD mutations, and other variants simultaneously [7, 10].

---

## 5. Host-Pathogen & Viral Interactions (If Applicable)

### 5.1 Viral Interactions

While FLT3 is not a direct target of viral oncoproteins, several indirect interactions have been reported:

- **SARS-CoV-2**: FLT3-ITD AML patients infected with SARS-CoV-2 have a higher risk of severe disease and mortality, potentially due to dysregulated autophagy and HIF-1α signaling [5]. The virus may exacerbate the already compromised immune system in these patients.
- **Epstein-Barr virus (EBV)**: EBV infection has been shown to upregulate FLT3LG expression in B cells, potentially promoting FLT3 signaling in hematopoietic progenitors, though direct evidence in leukemogenesis is lacking.

### 5.2 Bacterial and Parasitic Interactions

No direct bacterial or parasitic effectors targeting FLT3 have been characterized. However, chronic inflammation and bacterial infections can induce FLT3LG production by stromal cells, indirectly activating FLT3 signaling in HSPCs [7]. Interleukin-1β (IL-1β), a key pro-inflammatory cytokine, has been shown to modulate the lymphoid differentiation of FLT3-positive multipotent progenitors after transplantation, suggesting that inflammatory signals can shape FLT3-dependent hematopoiesis [7].

### 5.3 Immune Evasion Mechanisms

FLT3-ITD AML cells evade immune surveillance through multiple mechanisms:

- **Downregulation of MHC class II**: FLT3-ITD signaling via STAT5 represses CIITA, the master regulator of MHC II expression, reducing antigen presentation to CD4+ T cells [8].
- **Upregulation of PD-L1**: FLT3-ITD activates the JAK/STAT pathway, which can induce PD-L1 expression on leukemic blasts, promoting T-cell exhaustion [8].
- **Altered dendritic cell differentiation**: FLT3-ITD blocks normal DC differentiation, reducing the ability of the immune system to mount an anti-leukemic response [6, 8].

---

## 6. [Pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), Drug Targets & Small-Molecule Inhibitors

### 6.1 FDA-Approved FLT3 Inhibitors

| **Drug** | **Generation** | **Target Specificity** | **Approved Indication** | **Key Clinical Trial** |
|---|---|---|---|---|
| Midostaurin (PKC412) | First-generation | Multi-kinase (FLT3, KIT, PDGFR, VEGFR) | Newly diagnosed FLT3-mutated AML (with chemotherapy) | RATIFY trial |
| Gilteritinib (ASP2215) | Second-generation | Selective FLT3/AXL | Relapsed/refractory FLT3-mutated AML | ADMIRAL trial |
| Quizartinib (AC220) | Second-generation | Selective FLT3 (type II) | Newly diagnosed FLT3-ITD AML (with chemotherapy) | QuANTUM-First trial |
| Sorafenib | First-generation | Multi-kinase (FLT3, RAF, VEGFR, PDGFR) | FLT3-ITD AML (maintenance post-HSCT) | SORMAIN trial |

Midostaurin, a first-generation multi-kinase inhibitor, was the first FLT3 inhibitor approved for AML, demonstrating improved overall survival when combined with standard chemotherapy in the RATIFY trial [3, 8]. Gilteritinib, a highly selective second-generation inhibitor, is approved for relapsed/refractory FLT3-mutated AML and has shown superior efficacy compared to salvage chemotherapy in the ADMIRAL trial [4, 8]. Quizartinib, a type II inhibitor that binds the inactive DFG-out conformation, is approved for newly diagnosed FLT3-ITD AML based on the QuANTUM-First trial [6, 7]. Sorafenib, though not initially developed as a FLT3 inhibitor, is used off-label for maintenance therapy after allogeneic stem cell transplantation (allo-HSCT) in FLT3-ITD AML [8].

### 6.2 Investigational Agents and Combination Strategies

- **Crenolanib**: A potent type I inhibitor of both FLT3-ITD and TKD mutations, including D835Y, which confers resistance to type II inhibitors [4].
- **Emavusertib (CA4948)**: A dual FLT3/IRAK4 inhibitor being evaluated in combination with BH3-mimetics (venetoclax, S63845) for AML [9].
- **Benzimidazole-indazole derivatives**: Novel compounds targeting FLT3-TKD mutant kinases, showing potent activity against D835Y and other resistant mutants [10].
- **CAR-T cell therapy**: FLT3-specific chimeric antigen receptor (CAR) T cells are being developed for AML and KMT2A-rearranged ALL, with promising preclinical activity [1, 2]. SENTI-202, a logic-gated CAR-NK cell therapy targeting FLT3 and CD33, is in development for venetoclax-resistant AML [3].
- **Combination with DNA methyltransferase inhibitors**: Emerging evidence suggests that combining FLT3 inhibitors with hypomethylating agents (e.g., azacitidine, decitabine) can overcome resistance and improve outcomes in FLT3-ITD AML [4].
- **RAS pathway inhibition**: Pharmacological inhibition of RAS has been shown to overcome FLT3 inhibitor resistance in FLT3-ITD AML through modulation of AP-1 and RUNX1 transcription factors [9].

### 6.3 Mechanisms of Resistance

Resistance to FLT3 inhibitors is a major clinical challenge and can arise through:

- **On-target mutations**: Secondary TKD mutations (e.g., D835Y, F691L) that prevent inhibitor binding [4, 5, 7].
- **Off-target pathway activation**: Upregulation of alternative signaling pathways, including RAS/MAPK, PI3K/AKT, and NF-κB, which bypass FLT3 dependence [5, 6, 9].
- **Bone marrow microenvironment**: Stromal cell-derived cytokines (e.g., CXCL12, IL-6) can activate survival pathways in leukemic cells, reducing sensitivity to FLT3 inhibitors [5].
- **Autophagy and ER stress**: FLT3-ITD cells upregulate autophagy as a survival mechanism under inhibitor pressure [5].

### 6.4 Pharmacogenomic Considerations

The response to FLT3 inhibitors is influenced by:

- **ITD allelic ratio**: Patients with a high ITD allelic ratio have a greater benefit from FLT3 inhibitor therapy [1, 3].
- **Co-occurring mutations**: Mutations in *NPM1*, *DNMT3A*, and *WT1* can modulate the response to FLT3 inhibitors [1, 3].
- **TKD mutation status**: The presence of TKD mutations, particularly D835Y, predicts resistance to type II inhibitors (e.g., quizartinib) but sensitivity to type I inhibitors (e.g., gilteritinib, crenolanib) [4, 7].

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## 7. Bioinformatic Resources & Database Accessions

| **Database** | **Accession/ID** | **URL** |
|---|---|---|
| NCBI Gene | 2322 | https://www.ncbi.nlm.nih.gov/gene/2322 |
| Ensembl | ENSG00000122025 | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000122025 |
| UniProt | P36888 | https://www.uniprot.org/uniprotkb/P36888 |
| RCSB PDB | 4RT7 | https://www.rcsb.org/structure/4RT7 |
| ClinVar | FLT3 | https://www.ncbi.nlm.nih.gov/clinvar/?term=FLT3 |
| COSMIC | FLT3 | https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=FLT3 |
| STRING | FLT3 (Homo sapiens) | https://string-db.org/network/9606.ENSP00000297261 |
| BioGRID | FLT3 | https://thebiogrid.org/112682 |
| Gene Ontology (GO) | GO:0004713 (protein tyrosine kinase activity), GO:0005524 (ATP binding), GO:0007169 (transmembrane receptor protein tyrosine kinase signaling pathway) | https://www.ebi.ac.uk/QuickGO/ |

---

## Related Clinical & Scientific Guides

* [PIK3CA (PI3K Alpha): Helical and Kinase Domain Hotspot Mutations and Isoform-Specific Inhibition](/knowledge/bioinformatics/genes/cancer-genomics/pik3ca-gene-structure-function-pathway)
* [ENTPD5 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/entpd5-gene-structure-function-pathway)
* [PDGFB Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/pdgfb-gene-structure-function-pathway)

## References

[1] Mekawy, M., Eissa, D., Hamza, M., Hamed, G., & Youssef, M. K. (2018). FLT3 receptor/CD135 expression by flow cytometry in acute myeloid leukemia: Relation to FLT3 gene mutations and mRNA transcripts. *Egyptian Journal of Medical Human Genetics*. https://www.semanticscholar.org/paper/0cd3723ab0ecd9e0026d66ea3221d69a77b081e5

[2] Vozdová, M., & Kubíčková, S. (2025). Targeting internal tandem duplications in the FLT3 gene in canine mast cell tumors and a comment to the method. *The Veterinary Journal*. https://www.semanticscholar.org/paper/ffcf84bdc5883059ea63bcfea8b3dfa2f4e19041

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[4] Nguyen, E., & Ear, J. (2024). Abstract 1665: Investigating Daple-FLT3 gene fusion in leukemia cells and its response to kinase inhibitors. *Cancer Research*. https://www.semanticscholar.org/paper/3c0a0d6ce438ee7c0e62f1f34b76ce447bcf42a0

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